1. Field of the Invention
The present invention relates to a depth measurement apparatus for measuring the distance to an object, and particularly relates to a depth measurement apparatus that is used in an imaging apparatus or the like.
2. Description of the Related Art
In a digital still camera or a video camera, proposed is a solid image pickup device in which ranging pixels (depth measurement pixels) with a ranging function are arranged as a part or all of the pixels of the image pickup device, and the distance is detected based on the phase difference system (Japanese Patent Application Publication No. 2001-042462). The ranging pixels include a plurality of photoelectric conversion units. The plurality of photoelectric conversion units are disposed at positions that are substantially optically conjugate with the exit pupil of the camera lens via the microlens in the pixels. It is thereby possible to achieve a configuration where the light flux that has passed through different regions on the pupil of the camera lens can be guided to the respective photoelectric conversion units. Based on the signals obtained with the plurality of photoelectric conversion units disposed in each ranging pixel, an optical image (hereinafter referred to as the “image for ranging”) that is generated from the light flux that has passed through different pupil regions is thereby acquired. The distance can be measured by calculating the de-focus amount using the principle of triangulation based on the shift amount of the two images for ranging. Moreover, an imaging signal can be obtained by totaling the outputs of the plurality of photoelectric conversion units in one pixel.
In addition, in order to speed up the process of acquiring signals, known is a method of sharing a reading unit among the plurality of photoelectric conversion units, and adding and reading the outputs of the plurality of photoelectric conversion units. For example, known is a method of sharing the reading unit between two photoelectric conversion units, transferring the output of the first photoelectric conversion unit to an amplifying element and reading the output, thereafter transferring the output of the second photoelectric conversion unit to the amplifying element, and then reading the output sum of both photoelectric conversion units (Japanese Patent Application Publication No. 2004-134867). The output of the second photoelectric conversion unit is obtained by subtracting the output of the first photoelectric conversion unit from the output sum of both photoelectric conversion units. Consequently, in comparison to the method of individually transferring the output of the respective photoelectric conversion units to the amplifying element and then reading the output, the reading operation can be performed at a high speed since the number of reset operations of the amplifying element can be reduced.
Nevertheless, with the method described in Japanese Patent Application Publication No. 2004-134867, depending on the photographing conditions, there was a problem in that there is a region where the ranging accuracy in the plane of the image pickup device will deteriorate.
Generally speaking, the exit pupil position of the camera lens changes depending on the zoom or focus condition. Meanwhile, the positional relationship of the microlens and the photoelectric conversion unit in the pixel is fixed. Thus, depending on the photographing conditions, there are cases where the photoelectric conversion unit and the exit pupil deviate from a conjugate relation. When deviating from the conjugate relation, the regions on the pupil through which passes the light flux received by the respective photoelectric conversion units of the ranging pixels will differ according to the positions of the respective ranging pixels in the image pickup device. When the area of the light flux received with the respective ranging pixels becomes small on the pupil, the brightness of the detected image for ranging will deteriorate. Thus, the light intensity of the images detected with the respective photoelectric conversion units in the ranging pixels will differ according to the positions of the respective ranging pixels in the image pickup device.
Meanwhile, when the photoelectric conversion unit output is obtained based on subtraction, the SN ratio of the output signal (image signal for ranging) is low since the generation of random noise differs in comparison to the case of independently reading the photoelectric conversion unit output.
Even though the detected light intensity differed according to the positions of the ranging pixels in the image pickup device, conventionally, the output signal of the photoelectric conversion units of the same positional relationship in the ranging pixels was constantly read independently. Thus, there are cases where the photoelectric conversion unit with low detected light intensity and the photoelectric conversion unit (photoelectric conversion unit in which the output is obtained based on subtraction), which has a low SN ratio due to the subtraction, coincide, and the SN ratio of the image signal for ranging based on the output of this photoelectric conversion unit will deteriorate considerably. When the SN ratio of the image signal for ranging deteriorates, the reading error of the image deviation will increase, and the ranging accuracy will deteriorate. Since the detected light intensity depends on the positions of the respective ranging pixels in the image pickup device, there are regions on the plane of the image pickup device with a low ranging accuracy.
Note that, even in cases where the conjugate relation of the photoelectric conversion unit and the exit pupil of the camera lens is maintained, when there is shading of the light flux, or vignetting, in the lens frame, the detected light intensity will differ depending on the positions of the ranging pixels in the image pickup device. The change in light intensity will increase and the ranging accuracy will consequently deteriorate depending on the aperture diameter of the camera lens and the foregoing photographing conditions.